The production of gas, and in particular of hot air, at high temperature (which can be estimated a priori between 350.degree. and 450.degree. C.) with a high throughput (for example of the order of 10,000 to 30,000 Nm.sup.3 /h) is at present frequently brought about by a certain number of existing devices, in particular:
by direct heating of a flow of air by gaseous combustion products which are produced from a gas burner, the flow of air and the combustion products coming into contact and mixing; PA1 by indirect heating of the air by means of electrical resistances; PA1 by indirect heating of the air via a heat exchange with one or more heat-exchanging fluids heated by gas or fuel oil. PA1 the use of walls made of refractory material, which deteriorate very quickly as a result of operational deviations and successive restarting to which the device is subjected; PA1 the use likewise of special steels at the exchange stage between the combustion products leaving the burner and the air to be heated, the use of such steels nevertheless not preventing, in practice, frequent cracking of the walls of the exchangers, in view of the high value of the air/combustion products temperature gradient. PA1 to transfer by radiation the calorific energy contained in the combustion products, PA1 to cause the energy thus radiated to be absorbed by convection surfaces, PA1 and then to heat, essentially by convection, the said flow of fluid which is advantageously to be made to circulate in contact with the said convection surfaces. PA1 a first heat exchanger which has an internal volume through which a recycling pipe meanders, in which the combustion products for a heat exchange with the flow of fluid to be heated circulate, PA1 a second heat exchanger which has an internal volume in fluid communication with the internal volume of the said first exchanger and through which at least one conduit for heat-exchanging fluid runs, for a heat exchange with the flow of gaseous fluid in circulation in this second exchanger, PA1 and a third heat exchanger which also has an internal volume in fluid communication on the one hand with the volume of the said second exchanger and on the other hand with a conduit for the recovery of the heated gaseous fluid, at least one tube, which is provided for the circulation of combustion products in heat exchange with the said flow of gaseous fluid circulating in the third exchanger, meandering in the internal volume of this latter and being connected to the said recycling pipe for the combustion products.
It has, however, become apparent that these different existing systems have a certain number of disadvantages.
First, direct heating of a flow of air in contact with gaseous combustion products is proscribed for the manufacture of dietetic or pharmaceutical products, given the nature of the gases produced by mixing the combustion products and the air to be heated.
Electrical resistance heaters can only be economically used for approximately six months per year, given the high cost of electrical power during the coldest months of the year.
In the case of indirect heaters which function by heat exchange without direct contact between the combustion products and the fluid to be heated, although they represent the only devices (with the electric heaters) which allow indirect heating of the air to a temperature higher than approximately 300.degree. C., they nevertheless have certain disadvantages, of which the following can be noted:
Finally, existence can be noted of major drops in pressure resulting from the required compactness of these exchanges.